Why don’t fish freeze in cold water?

From the course of the school curriculum, we already know that fish are cold-blooded creatures. This means that their body temperature is equal to the temperature of the water in which they swim. This raises the question: how do fish that swim in the cold waters of Antarctica not die from hypothermia? After all, the water temperature in these regions is quite extreme and rarely rises above 4 degrees Celsius. Considering that the freezing point of seawater is -2 degrees while swimming in icy waters, fish should also turn into ice. As it turned out, the blood of some fish species contains a kind of antifreeze – a substance that does not freeze at subzero temperatures. How fish adapt to the extreme conditions of Antarctica has been elucidated in a study sponsored by German car manufacturer Volkswagen.

Fish of Antarctica

As a rule, the so-called nototheniform fish swim in Antarctic waters. According to the calculations of scientists, today there are about 156 species. Many of them are eaten and even imported into Russia. For example, sea pike perch (Patagonotothen ramsayi) or ice fish (Champsocephalus gunnari) can be found in the sale. People from the times of the USSR can also recall that on the market one could find marble notothenia (Notothenia rossii) and a couple of other fish caught from the cold waters of Antarctica.

As mentioned above, notothenium fishes live in waters where the temperature practically does not rise above 4 degrees Celsius. Although, lately, the weather in the coldest region of our planet has been downright weird. In early 2020, I published the news that a record high air temperature was recorded in Antarctica – 18.3 degrees Celsius. She probably influenced the state of the water and the fish living in it, apparently, felt great.



Amazing abilities of fish

But let’s get back to the fact that most of the time fish spend in cold water, at a depth of about 1500 meters. In the course of studying fish organisms, scientists have found that the transformation of their bodies into blocks of ice is prevented by special proteins called glycoproteins. Back in the middle of the 20th century, scientists found out that these substances inhibit the freezing of liquids 200-300 better than salt, which is used to cover the roads of our country’s cities in winter. This is a kind of antifreeze that allows fish to withstand extremely low temperatures.

Proteins that save the lives of fish act not only inside their body but also have an external influence on the water. In the course of scientific work, the researchers found that in the presence of glycoproteins in the blood of fish, the water molecules surrounding their bodies begin to move more slowly, which prevents the formation of ice crystals. That is, fish have no risk of turning into blocks of ice – this is simply impossible.

Warm-blooded fish

But not all fish are resistant to cold waters. One of them is a one-of-a-kind warm-blooded fish, the red-finned fish (Lampris guttatus). At the beginning of the article, I mentioned that all fish are cold-blooded and their body temperature directly depends on the temperature of the water. So, the red-finned opah is an exception to this rule. In 2015, American scientists found out that their body temperature is usually 5 degrees higher than the water temperature. And all because this fish constantly warms itself up, actively moving its fins. And their thick fat layer helps to retain heat inside the body.